A Highly Durable Rubber‐Derived Lithium‐Conducting Elastomer for Lithium Metal Batteries

Abstract Elastomers offer attractive advantages over classical solid‐state electrolytes in terms of ensuring stable interfacial contact and maintaining fatigue durability, but the low ionic conductivity obstructs their practical applications in long‐life lithium metal batteries. In this work, rubber‐derived lithium‐conducting elastomer has been developed via sulfur vulcanization of nitrile butadiene rubber with a polymerizable ionic liquid to provide both high resilience and dramatically improved ionic conductivity. Owing to the chemically crosslinked network between rubber chains and ionic liquid fragments generated during vulcanization, the elastic lithium‐conductor achieves high resilience of 0.92 MJ m−3, superior cyclic durability of 1000 cycles at 50% strain, and high room‐temperature ionic conductivity of 2.7 × 10−4 S cm−1. Consequently, the corresponding solid‐state lithium/LiFePO4 battery exhibits a high capacity of ≈146 mAh g−1 with a high capacity retention of 94.3% for up to 300 cycles.

2 vulcanizing at 180 o C for 1 h under nitrogen atmosphere. It is noted that after evaporation of solvents, the slurry yields a gel electrolyte denoted NBR-IBIL gel electrolyte. For comparison, NBR and vulcanized NBR (denoted v-NBR) electrolytes were also prepared in the same procedures except that IBIL/sulfur agent or IBIL was not added to the precursor slurries, respectively. NBR, and NBR/IBIL hybrid membranes without LiTFSI were also fabricated to determine the changes in element valence before and after vulcanization.
Material Characterizations: The morphology and element distribution of NBR-based membranes were obtained from a field emission scanning electron microscope (FESEM, JSM-7500). X-ray diffraction (XRD) patterns were characterized on a Rigaku D/Max 2500 X-ray diffractometer with CuKα radiation. Fourier transform infrared spectroscopy (FTIR) spectra of NBR-based membranes were measured on a Nicolet Nexus 670 spectrometer in attenuated total reflection (ATR) mode. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out using DSC-Q20 and TGA-Q50 from TA Instruments at a ramp rate of 10 o C in air, respectively. Chemical compositions and elemental states of NBR-based membranes were investigated using Thermo Fisher Scientific ESCALAB 250 with Al Kαradiation. Binding energy values of C1s peaks were calibrated to be 284.8 eV. Solid-state 7 Li NMR spectra of electrolyte membranes were measured in a double resonance 3.2 mm magic angle spinning probe on a JNM-ECZ600R spectrometer with a spinning frequency of 12 kHz and a single pulse sequence, and the 7 Li shifts were referenced to LiCl (-1.19 ppm). Tensile strength and creep tests were performed on DMA Q800 (TA Instruments) at a strain rate of 100% min -1 and step stress of 0.1 MPa at 25 o C, respectively.
Electrochemical Measurements: Ionic conductivities of elastic electrolytes were calculated 3 using the following equation: where d was the thickness, Rb was the bulk resistance, and A was the area of elastic electrolytes.
Rb of elastic NBR-based electrolytes was determined at room temperature by assembling electrolytes discs (Φ, ~16 mm; thickness, ~120 μm) in 2032 coin cells sandwiched between two steel spacers and using electrochemical impedance spectroscopy (EIS) on a CHI 760E electrochemical workstation (Chenhua, Shanghai) in the frequency range of 1 MHz to 10 mHz with an amplitude of 5 mV. Li ion transference numbers (tLi + ) of NBR/IBIL hybrid and NBR-IBIL gel electrolytes were measured at 25 o C using potentiostatic polarization method proposed by Bruce and Appetecchi [1][2] . After applying a polarization potential of 10 mV (ΔV), tLi + in the symmetrical battery system can be calculated using the following equation: where I0 is the initial current, Iss is the steady-state current, and R0 and Rss are the corresponding initial and steady state resistances, respectively.  electrolyte, implying that lithium salt is uniformly distributed in the hybrid electrolyte [3][4] .
Furthermore, NBR/IBIL hybrid electrolytes exhibit XRD patterns similar to those of NBR electrolytes, but the intensity is less than two-thirds that of the NBR electrolyte under the same test conditions, revealing that grafting IBIL fragments may increase the amorphous regions of NBR/IBIL hybrid electrolyte, thus facilitating local segmental motions of polymer.
7 Figure S4. demonstrate that NBR/IBIL hybrid matrix has been successfully fabricated. Moreover, the -C=C-peak intensity of the hybrid matrix centered at 970 cm −1 is significantly lower than that of NBR and IBIL, demonstrating that large consumption of -C=C-bonds occurred during vulcanization. Noted that a broad water peak can be observed at 3562 cm −1 because the electrolyte was exposed to air for a long time before the test, which would be avoided by quickly transferring to the glove box after sulfur vulcanization.      cycles. The electrolyte has a low hysteresis at the initial cycles, which remain at low levels even after 1000 cycles. 13 Modulus of resilience (Ur) is the maximum reversible stored energy in the material and can be described as an area under the linear stress-strain curve. It can be calculated by the following equation: where E is the Young's modulus, σy is the yield strength, and εy is the yield strain. If the yield point cannot be clearly observed, stress-strain curve is approximated as linear; thus, tensile strength and elongation at break are used as σy and εy to calculate Ur from reference, respectively.          (b-d) Digital pictures of the solid-state pouch cell connected to the LED being bent, punctured, and cut. The solid-state pouch cell assembled from metallic lithium anode, NBR/IBIL hybrid electrolyte, and standard LFP cathode operates well at ambient temperature, even when the pouch cell is bent, punctured, and cut, proving that NBR/IBIL hybrid electrolyte can ensure the safety and stability of its pouch cells during operating.